A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Lithographic apparatus use a radiation beam to illuminate the patterning device, and thereby provide patterned radiation which is projected onto the substrate. Properties of the radiation beam are selected in order to provide a high quality image at the substrate. One of the properties of the radiation beam which may be selected is the angular distribution of radiation within the radiation beam. This property is conventionally referred to as the illumination mode. Illumination modes which are commonly used include annular, dipole, and quadruple modes.
The illumination mode which is selected when projecting a given pattern onto a substrate will depend upon properties of that pattern. It is desirable therefore for a lithographic apparatus to be able to switch between illumination modes, in order to allow the lithographic apparatus to accurately project different patterns onto a substrate. In some instances it may be desirable to switch rapidly between illumination modes. For example, if two patterns are being consecutively imaged onto a given substrate (an approach known as double patterning), then it may be desirable to use a first illumination mode when projecting the first pattern onto the substrate, and switch to a second illumination mode when projecting the second pattern onto the substrate. Switching rapidly between the first and second illumination modes allows the lithographic apparatus to have a higher throughput.
In order to be able to rapidly switch between illumination modes, a lithographic apparatus may be provided with an illumination mode selector which includes an array of mirrors (or other reflective elements) that may be used to select different illumination modes.